Why the water's edge matters most

The strip where water meets land is the busiest few feet in any garden. Damselfly nymphs climb emergent stems to split their skins and dry their wings. Froglets lurk in the tangle before venturing onto the lawn. Hoverflies, bees and moths work the flowers, and the seedheads feed finches long after the colour has gone.

Marginal plants do two jobs at once. They blur the hard line between water and bank so the pond or brook looks like it belongs there, and they create the cover that wildlife needs to move safely between the two. A pond without marginals is a hole with a plastic rim. A brook without them is a drain.

Know your waterline before you plant

"Marginal" covers a wide range of conditions, and most disappointments come from putting a plant at the wrong depth. Think in three rough zones.

• Bog garden: soil permanently damp, water table just below the surface, no standing water over the crown. Ragged robin, meadowsweet and marsh marigold belong here.
• Shallow marginal: up to about 15 cm of water above the soil surface. Water mint, water forget-me-not and lesser spearwort thrive at this depth.
• Deeper shelf: 15–40 cm of water. Yellow flag iris, flowering rush and purple loosestrife cope comfortably.

In a lined pond, plant into open-weave baskets so roots can escape and the crown stays at the right level. On a natural brookside bank, plant straight into damp silt and let the water table do the work.

Native marginals for colour through summer

Aim for a sequence rather than a single flush. These all earn their place and are native or long-established in the UK.

• Water forget-me-not (Myosotis scorpioides) — low, sprawling and blue from late spring, ideal for softening the front edge of a shelf.
• Water mint (Mentha aquatica) — lavender pom-poms from July, and one of the best nectar plants you can put by water.
• Purple loosestrife (Lythrum salicaria) — magenta spires from July to September that hum with bees. Vigorous, so keep it in a basket or thin it every couple of years.
• Flowering rush (Butomus umbellatus) — neat pink umbels in late summer on a plant that behaves itself.
• Ragged robin (Lychnis flos-cuculi) — ragged pink stars for a damp meadow strip rather than open water.
• Meadowsweet (Filipendula ulmaria) — creamy froth with an almond scent that pulls in small flies and bees by the dozen.

Structure, seedheads and winter cover

Flowers are only half the story. Sedges, rushes and grasses give the planting its bones, and their hollow stems are where a great many insects spend the winter. The caterpillars of several moths feed on sedge and rush foliage, and the tussocks make snug cover for frogs.

• Great pond sedge (Carex riparia) — bold, arching, superb at the back of a marginal shelf.
• Soft rush (Juncus effusus) — tight evergreen clumps that hold the bank and shelter amphibians.
• Pendulous sedge (Carex pendula) — graceful drooping seedheads, and tolerant of drier shade further up the bank.
• Water plantain (Alisma plantago-aquatica) — whorled seedheads that stand well into winter.

Resist the urge to tidy in autumn. Leave stems standing until late March, then cut back and rest the cuttings on the bank for a day or two so anything sheltering can crawl back to the water.

Planting and settling in

Late April to early June, or September, are the kindest windows. Use a loam-based compost rather than multi-purpose: it holds together, sinks instead of floating and releases far fewer nutrients into the water. Top each basket with gravel to stop fish and birds rooting about, and add a stone for weight. Two or three plants per basket is plenty.

Don't feed marginals. Rich water grows blanketweed, not flowers. Give new plants a full season to settle, and leave one shallow, gravelly slope clear of planting as a drinking and bathing spot for birds and hedgehogs.

Brookside banks, flow and shelter

If your garden runs along a brook, plant the bank and leave the channel alone. A row of tall stems across the flow will dam debris, push water into the bank and cause the erosion you were trying to prevent. Keep the centre clear and work outwards.

Put sedges and rushes on the lowest, wettest ground, where their roots will bind the soil. Set meadowsweet, loosestrife and ragged robin a little further back where the ground is damp rather than sodden. Leave a rough, unmown strip along the top of the bank — that scruffy ribbon is what water voles, grass snakes and overwintering insects actually use.

Check before you alter a bank or watercourse, as permission is often needed, and never tip pond or bank cuttings into the brook. A few well-chosen marginals, planted at the right depth and left standing through winter, will give you colour for months and a living edge that keeps on giving.